1. Field of the Invention
This invention relates to a center electrode structure having an oxidation, heat resistant enclosure and an electrically conductive core member encased into the enclosure, and paticularly concerned to an improved center electrode structure so as to advantageously absorb thermal deformation due to thermal expansional difference therebetween, under high ambient temperature.
2. Description of the Prior Art
In a spark plug for an internal combustion engine, a center electrode exposes one end to a combustion chamber, so that it is subjected to a large quantity of heat and oxidation for an extended time period.
To endure against the adverse heat and oxidation environment, a center electrode is made from a copper-based core and a platinium or nickel-based enclosure clad by means of extrusion to ensure good electrical conduction, oxidation and heat resistivity simultaneouly. By way of illustration as taught by Japanese Patent No. 20614/1971 now public, the enclosure is previously purged and annealed for one hour at 650 degrees centigrade, at the same time, the core is also purged and pickled. The core thus pickled, is pressure fit into the enclosure in the air-tight relationship, and placed under the inert gaseous atmosphere at the temperature of 930 degrees centigrade for 1.5 hours so as to allow diffusion between the core and the enclosure.
The center electrode thus comprised, however, renders the platinium-based enclosure expensive, although advantageous in endurance and machining. In addition to it, the center electrode is subjected to deformation due to thermal stress between the core and the enclosure, thus leading to deviating from a normal discharge gap so as to be short of good and stable sparking action.
Therefore, it is an object of this invention to provide a center electrode structure which is capable of absorbing a thermal expansional difference between a core and enclosure members, protecting against inadvertent deformation to ensure good and stable sparking action for an extended time period with relatively low cost.
According to the invention, in a center electrode structure for spark plug comprising a tublar insulator, the inner side of which has a stepped shoulder to provide a diameter-reduced bore therein; a center electrode having a flange, and concentrically placed into the insulator with the flange engaging the shoulder, and with one end being axially through the bore to be exposed to outside of the insulator; the center electrode having an electrically conductive core member of copper or copper-based alloy which is encased into an enclosure made of an oxidation and heat resistant nickel-based alloy; and said core member being provided with a number of fractures in its granular structure so as to absorb thermal stress due to the thermal expansional difference between the core and enclosure members.